1. Field of the Invention
The present invention relates to wafer inspection and in particular to a multi-spot scanning technique using a spot array having gaps between the spots to achieve high speed and high sensitivity wafer inspection with minimum cross-talk between spots.
2. Related Art
Many prior art inspection systems have used a single spot to scan a wafer surface. Unfortunately, the data rate of a single channel (i.e. single spot) is typically limited to be less than 200 megapixels per second (MPPS). However, for the next generation inspection systems, the total data rate is required to be more than 10 gigapixels per second (GPPS), which requires at least 50 channels.
Various multi-spot inspection systems have been proposed to overcome the limited data rate of the single spot inspection systems. For example, U.S. Pat. No. 6,236,454 teaches a multi-beam inspection system 100, which is shown in FIG. 1. Multi-beam inspection system 100 includes a multi-beam laser scanning system 101 that generates multiple beams (and thus multiple spots) that are scanned across a sample (e.g. a wafer) 107 and a multi-beam imaging system 102 that collects the light scattered by sample 107 from such scanning. In general, multi-beam laser scanning system 101 includes a beam generator 103 (multiple lasers or a single laser with multiple beam splitters) to generate the multiple beams, pre-scan optics 104 that provide the beams with the desired optical properties, one or more scan units 105 that deflect the beams to provide the scanning motion, and an objective lens 106 that focuses the scanning beams on sample 107.
Due to the motion of scan unit(s) 105, the focused beams move in a first direction. Typically, sample 107 is moved in a second direction orthogonal to the first direction. The first and second directions allow inspection system 100 to provide two-dimensional scanning. The scanning rate (e.g. spots/sec) is a function of the spot velocity and spot size (both functions of scan unit(s) 105).
Multi-beam imaging system 102 includes collection optics 108 and photodetectors 109. Collection optics 108 can be a single lens or multiple optical components. Photodetectors are placed in an image plane near the location where the scan lines of the scanned beams are imaged by collection optics 108.
FIG. 2 illustrates two scans 201A and 201B performed on a wafer surface 200. Note that each scan 201A and 201B is formed by a plurality of first movements 202 (due to scan unit(s) 105) and a plurality of second movements 203 (due to the moving of sample 107). The first movements 202 define a scan line comprising a plurality of spots, each spot having a scan field. Note that any gaps in the scan area (e.g. the gap between scans 201A and 201B) can be filled by using another round of scan.
Specifically, in multi-beam inspection system 100, the optical field of view (FOV) is equally divided by the number of spots, and the width of the scan field of each spot is one-half of the divided optical field. Such arrangement ensures that the residual light scattered from one spot does not enter the collection channels of the other spots, thereby allowing a clean separation of the spots at photodetectors 109 (i.e. the detector plane).
Unfortunately, multi-beam inspection system 100 has two significant limitations. As a first disadvantage, the effective field of view (FOV) is a factor N/(2N−1) of the available FOV of the scan optics, where N is the number of spots. When N is large, the effective FOV is approximately only one-half of the FOV of scan optics 106 (i.e. the objective lens). That is, at any point in time during the scan, only one-half of the FOV is being used. (Note that although the FOV in FIG. 2 is shown as being 3× the scan line length for 2 spots, actual FOVs in actual wafers could be much wider. Thus, the FOV in FIG. 2 is merely to emphasize the use ratio.) For higher resolution inspection, a large objective lens is used to provide the FOV. This large objective lens is difficult and expensive to manufacture. Thus, the above-described scanning effectively wastes the actual FOV (provided by a high resolution objective lens).
As a second disadvantage, the FOV of the objective lens 106 is physically limited, especially at very high resolution. As a result of the physical limitations of the optics, the scan field of each spot decreases as the number of spots increases, which for any given data rate results in the increase of both line frequency (i.e. the first movements 202) and the stage speed (i.e. the second movements 203). Unfortunately, this increase in line frequency and stage speed requires very expensive electronics and is subject to the physical limits of stage speed and scanner frequency. Therefore, multi-beam inspection system 100 is typically limited to a small number of spots (for example, less than 10). However, as noted above, this limited number of spots (i.e. channels) is not suitable for high data rate (high speed) inspection.
U.S. Pat. No. 6,636,301, issued to KLA-Tencor, teaches a method of multi-beam inspection that eliminates the inefficiency of using the optics FOV by offsetting the spots in two directions. Specifically, as shown in FIG. 3A, spots 301A, 301B, and 301C are offset in both vertical and horizontal directions from each other. Note that each spot 301A, 301B, and 301C has a scanning length L that forms a scanning stripe 302A, 302B, and 302C, respectively (in the vertical direction). The scanning stripes 302A, 302B, and 302C form a swatch S. In the vertical direction, spots 301A, 301B, and 301C are offset by approximately L (e.g. scanning length L minus an overlap portion O). In the horizontal direction, spots 301A, 301B, and 301C are offset by distance W.
As shown in FIG. 3B, spots 301A, 301B, and 301C, and more specifically stripes 302A, 302B, and 302C, are moved across a wafer surface in serpentine patterns 310 (solid line), 311 (dashed line), and 312 (dotted line), respectively. Notably, the described offsets allow the spots to be separated at the detector plane without leaving gaps between the scan fields of each spot. Although this method eliminates the inefficiency of utilizing the optics FOV, it still divides the optical FOV among the spots, which is subject to the limitation of scanner line rate and stage speed. Therefore, this inspection system with both vertical and horizontal spot offset is also typically suitable for only a small number of spots.
U.S. Pat. No. 7,049,155 teaches a multi-beam inspection system that uses a scan pattern non-perpendicular to the wafer movement. Specifically, FIG. 4 illustrates five scans of a multi-beam scan pattern with four beams, thereby generating 20 scan lines 401 (return scan lines 402 shown for reference). Notably, the scan pattern is not perpendicular to a direction S (i.e. the movement of the wafer). Note that D represents the distance between scan lines 401, whereas T represents the distance between scan lines in the mechanical scanning direction.
FOV 403 represents the horizontal field of view (FOV) of the multi-beam inspection system, which like other prior art systems, divides its FOV by the number of spots (in this case, four spots) and is, therefore, limited to having a small number of spots. Note that the separation of spots at the detectors can be achieved by tilting the scan direction away from the perpendicular to direction S. However, the angle of tilt is also determined by the scan line rate and stage speed, which has limited flexibility. Therefore, this multi-beam inspection system also has a number of significant disadvantages.
U.S. Pat. No. 7,130,039 teaches a multi-spot inspection imaging system that uses an array of illuminated spots. FIG. 5A illustrates an exemplary multi-spot array 501 that is slightly rotated with respect to the tangential direction Q of the wafer as the wafer is rotated. Note that the spots in multi-spot array 501 “paint” adjacent tracks. For example, FIG. 5B illustrates adjacent spots 502 and 503 traveling along tracks 504 and 505, respectively. Tracks 504 and 505 may be offset by a separation equal to one-third or one-quarter of the spot size to achieve a desired sampling level (e.g. 3×3 or 4×4 samples per spot width) (thus, spots 502 and 503 overlap by two-thirds or three-quarters of the spot size). Thus, a 1D scan of the wafer produces a 2D image with no gap between tracks. For inspection analysis, the resulting cross-talk from this scan must be “undone” to separate the spots at the detector plane.
As demonstrated from the above-described inspection systems, although the generation of multiple spots to illuminate a sample (e.g. a wafer) is relatively straightforward, such systems typically limit the number of spots, the speed of the scanner, and/or the speed of stage to yield accurate results. Therefore, a need arises for a multi-spot inspection system that can provide a high data rate commensurate with the next generation of inspection system requirements.